1. Field of the Invention
This invention relates to a purging system and a purging method for use when purging the interior of a space in a portable type hermetically sealed container, and more particularly to a portable type hermetically sealed container such as an SMIF pod which is a portable type hermetically sealed container for use when conveying a reticle or the like, for example, in the manufacturing process of a semiconductor or the like, and to a purging system for and a purging method of carrying out the process of purging the interior of a container such as the SMIF pod.
2. Related Background Art
Silicon wafers, glass substrates for liquid crystal display, substrates for hard disks, reticles, etc. are usually kept in a portable type hermetically sealed container, and are transported between processing apparatuses in manufacturing processes conforming to respective purposes while being kept in the hermetically sealed container. It is known that when these substrates, etc. are held in the atmosphere, natural oxidated film is formed on the surfaces thereof or the adherence of organic substances to the surfaces thereof occurs. This phenomenon often poses a problem in subjecting the substrates, etc. to some processing and therefore, the interior of the aforementioned hermetically sealed container is usually filled with an inert gas such as nitrogen of high purity, and contains and keeps the substrates, etc. in this state.
The hermetically sealed container has its hermetically sealed state released when the substrates, etc. therein are taken out or the substrates are put thereinto. This releasing operation is performed under a highly clean environment, and this environment is one formed in the so-called atmosphere to the last. Accordingly, it is necessary to perform the so-called purging operation of exhausting the atmosphere (gas) introduced into the interior of the container after the container has been hermetically sealed and letting nitrogen or the like which is an inert gas flow thereinto. This purging operation is in some cases performed at a position whereat the putting of the substrates, etc. into and out of the container is effected, but when the substrates, etc. are large-sized, this operation is often performed in a system exclusively for purging with the purging efficiency taken into account.
The epitome of a conventional purging system is shown by way of example in Japanese Patent Application Laid-Open No. H7-86370 (see FIGS. 8 and 9 thereof and the item of the prior art). In this purging system, there are provided an inert gas introduction route and an exhaust route having openable and closable valves capable of communicating with the interior of a container. This container is installed in the purging system, and an intake valve in the introduction route and an exhaust valve in the exhaust route are opened to thereby effect the discharge of a gas present in the interiors of the container and the system and the introduction of an inert gas thereinto. Also, in a case where the substrates, etc. are preserved in the interior of this container for a long period, it is necessary to perform the operation of purging the interior of the container at suitable time intervals from the viewpoint of the quality control of the substrates, etc.
In this purging system, the inert gas to be supplied to the introduction route is maintained at predetermined pressure, and in the exhaust route, the exhaust amount thereof is made adjustable by the opening and closing of the exhaust valve. By effecting the adjustment of this exhaust amount, it is made possible to increase the pressure in the interior or the like of the container to a predetermined value, and after the increase in the pressure, the exhaust value is suitably increased to thereby achieve a reduction in the amount of consumed inert gas in the purging operation.
Also, in Japanese Patent No. 3191392, in a container left in a state containing substrates, etc., as a countermeasure for a case where an inert gas enclosed therein has leaked out of the container, there is disclosed a construction using a so-called pressure reducing valve as an intake valve in a gas introduction route. According to this construction, when the pressure in the interior of the container connecting to the secondary side of the pressure reducing valve assumes a predetermined value or less, an inert gas of predetermined pressure is introduced from the primary side of the pressure reducing valve to thereby keep the interior of the container at predetermined pressure.
As described above, in the purging operation for the portable type hermetically sealed container, the opening and closing of the intake valve for effecting the gas supply from a supply source of the inert gas of the predetermined pressure and the amount of opening and closing of the exhaust valve provided in the exhaust route are suitably adjusted to thereby effect the control of the pressure in the interior or the like of the container. Also, it is known to make the intake valve capable of adjusting pressure to thereby effect the control of the pressure in the interior or the like of the container.
Here, it is confirmed by the applicant that the pressure in the interior or the like of a container repeats the fluctuation of increase or decrease with this opening and closing operation. Accordingly, when the outflow of an inert gas or the like is controlled by the opening and closing of a valve, it seems to be difficult to accurately keep the pressure in the interior or the like of the container constant. Also, even in a case where as an exhaust valve, use is made of valve opening controllable one and it is controlled on the basis of the pressure in the interior or the like of the container, if the introduction of an inert gas is effected by the opening and closing of the valve, the pulsation of the inert gas is caused by this operation as has heretofore been experienced. This pulsation causes a fine pressure fluctuation at short time intervals in the interior of the container. This tendency is considered to become more remarkable as a gas supplying speed necessary for the container to become large becomes higher. Also, even if the intake valve is interchanged with a pressure reducing valve capable of adjusting pressure, pressure adjustment is basically effected by the opening and closing of an inert gas introduction valve, and it seems to be difficult to stabilize the pressure.
This fine pressure fluctuation occurring at short time intervals gives an unnecessary load to a seal member used to connect the interior of the container and the space in the purging system together as a hermetically sealed space, and may cause the deterioration of the seal member. Also, when the pressure fluctuation becomes extremely great, this seal member becomes incapable of completely keep a hermetically sealing state, and this may cause the leakage of the inert gas from the seal portion. Also, it is conceivable as a possibility that with the pressure fluctuation, vibration is given to substrates, etc. held in the interior of the container to thereby produce unexpected dust or the like.